Copyright ©2000 W3C® (MIT, INRIA, Keio), All Rights Reserved. W3C liability, trademark, document use and software licensing rules apply.
XML Schema Part 0: Primer is a non-normative document intended to provide an easily readable description of the XML Schema facilities and is oriented towards quickly understanding how to create schemas using the XML Schema language. XML Schema Part 1: Structures and XML Schema Part 2: Datatypes provide the complete normative description of the XML Schema definition language, and the primer describes the language features through numerous examples which are complemented by extensive references to the normative texts.
The XML Schema Part 0: Primer is a part of the W3C XML Activity.
This is a public working draft of XML Schema 1.0 for review by the public and by members of the World Wide Web Consortium. The XML Schema Working Group has agreed to its publication. Note that some sections of this draft may not be up-to-date with the XML Schema language described in Parts 1 and 2 of the XML Schema specification. Known discrepancies are noted in the text.
The Working Group does not anticipate further substantial changes to the syntax described here, although this is still a working draft, and is subject to change based on experience and on comment by the public, and other W3C working groups.
A list of current W3C working drafts can be found at http://www.w3.org/TR/. They may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use W3C Working Drafts as reference material or to cite them as other than "work in progress".
2 Basic Concepts: The Purchase Order
2.1 The Purchase Order Schema
2.2 Complex Type Definitions,
Element & Attribute Declarations
2.3 Simple Types
2.3.1 List Types
2.3.2 Union Types
2.4 Anonymous Type
Definitions
2.5 Element Content
2.5.1 Complex Types from
Simple Types
2.5.2 Mixed Content
2.5.3 Empty Content
2.6 Annotations
2.7 Building Content Models
2.8 Attribute Groups
2.9 Null Values
3. Advanced Concepts I: Namespaces, Schemas
& Qualification
3.1 Target Namespaces &
Unqualified Locals
3.2 Qualified Locals
3.3 Global vs. Local
Declarations
3.4 Undeclared Target
Namespaces
4. Advanced Concepts II: The International
Purchase Order
4.1 A Schema in Multiple
Documents
4.2 Deriving Types by
Extension
4.3 Using Derived Types in
Instance Documents
4.4 Deriving Complex Types by
Restriction
4.5 Redefining Types and Groups
4.6 Substitution Groups
4.7 Abstract Elements and
Types
4.8 Preventing the
Creation and Use of Derived Types
5. Advanced Concepts III: The
Quarterly Report
5.1 Specifying
Uniqueness
5.2 Defining Keys
and their References
5.3 XML Schema
Constraints vs. XML 1.0 ID Attributes
5.4 Importing Types
5.4.1 Type Libraries
5.5 Any Element, Any Attribute
5.6 schemaLocation
5.7 Conformance
A. Acknowledgements
B. Simple Types & Their
Facets
C. Regular Expressions
D. Index
E. Document History
This document, XML Schema Part 0: Primer, provides an easily approachable description of the XML Schema definition language, and should be used alongside the formal descriptions of the language contained in Parts 1 and 2 of the XML Schema specification. The intended audience of this document includes application developers whose programs read and write schema documents, and schema authors who need to know about the features of the language, especially features that provide functionality above and beyond what is provided by DTDs. The text assumes that you have a basic understanding of XML 1.0 and XML-Namespaces. Each major section of the primer introduces new features of the language, and describes those features in the context of concrete examples.
Section 2 covers the basic mechanisms of XML Schema. It describes how to declare the elements and attributes that appear in XML documents, the distinctions between simple and complex types, defining complex types, the use of simple types for element and attribute values, schema annotation, a simple mechanism for re-using element and attribute definitions, and null values.
Section 3, the first advanced section in the primer, explains the basics of how namespaces are used in XML and schema documents. This section is important for understanding many of the topics that appear in the other advanced sections.
Section 4, the second advanced section in the primer, describes mechanisms for deriving types from existing types, and for controlling these derivations. The section also describes mechanisms for merging together fragments of a schema from multiple sources, and for element substitution.
Section 5 covers more advanced features, including a mechanism for specifying uniqueness among attributes and elements, a mechanism for using types across namespaces, a mechanism for extending types based on namespaces, and a description of how documents are checked for conformance.
In addition to the sections just described, the primer contains a number of appendices that provide detailed reference information on simple types and a regular expression language.
The primer is a non-normative document, which means that it does not provide a definitive (from the W3C's point of view) specification of the XML Schema language. The examples and other explanatory material in this document are provided to help you understand XML Schema, but they may not always provide definitive answers. In such cases, you will need to refer to the XML Schema specification, and to help you do this, we provide many links pointing to the relevant parts of the specification. More specifically, XML Schema items mentioned in the primer text are linked to an index of element names and attributes, and a summary table of datatypes, both in the primer. The table and the index contain links to the relevant sections of XML Schema parts 1 and 2.
The purpose of a schema is to define a class of XML documents, and so the term "instance document" is often used to describe an XML document that conforms to a particular schema. In fact, neither instances nor schemas need to exist as documents per se -- they may exist as streams of bytes sent between applications, as fields in a database record, or as collections of XML Infoset "Information Items" -- but to simplify the primer, we have chosen to always refer to instances and schemas as if they are files.
Let us start by considering an instance document in a file
called po.xml. It
describes a purchase order generated by a home products
ordering and billing application:
The purchase order consists of a main element,
purchaseOrder, and the subelements
shipTo, billTo, and items.
These subelements in turn contain other subelements, and so
on, until a subelement such as USPrice
contains a number rather than any subelements. Elements
that contain subelements or carry attributes are said to
have complex types, whereas elements that contain numbers
(and strings, and dates, etc) but do not contain any
subelements are said to have simple types. Some elements
have attributes; attributes always have simple types.
The complex types in the instance document, and some of the simple types, are defined in the schema for purchase orders. The other simple types are defined as part of XML Schema's repertoire of built-in simple types.
Before going on to examine the purchase order schema, we digress briefly to mention the association between the instance document and the purchase order schema. As you can see by inspecting the instance document, the purchase order schema is not mentioned. An instance is not actually required to reference a schema, and although many will, we have chosen to keep this first section simple, and to assume that any processor of the instance document can obtain the purchase order schema without any information from the instance document. In later sections, we will introduce explicit mechanisms for associating instances and schemas.
The purchase order schema is contained in the file
po.xsd:
The purchase order schema consists of a schema element and a variety
of subelements, most notably element, complexType, and simpleType which
determine the appearance of elements and their content in
instance documents.
Each of the elements in the schema has
a prefix xsd: which is associated with the XML
Schema namespace through the declaration,
xmlns:xsd="http://www.w3.org/2000/08/XMLSchema", that
appears in the
schema element. The prefix xsd: is
used by convention to denote the XML Schema namespace,
although any prefix can be used. The same prefix, and hence
the same association, also appears on the names of built-in
simple types, e.g. xsd:string. The purpose of the association
is to identify the elements and simple types as belonging
to the vocabulary of the XML Schema language rather than
the vocabulary of the schema author. For the sake of
clarity in the text, we just mention the names of elements
and simple types (e.g.
simpleType), and omit the prefix.
In XML Schema, there is a basic difference between complex types which allow elements in their content and may carry attributes, and simple types which cannot have element content and cannot carry attributes. There is also a major distinction between definitions which create new types (both simple and complex), and declarations which enable elements and attributes with specific names and types (both simple and complex) to appear in document instances. In this section, we focus on defining complex types and declaring the elements and attributes that appear within them.
New complex types are defined using
the
complexType element and such definitions
typically contain a set of element declarations, element
references, and attribute declarations. The declarations
are not themselves types, but rather an association between
a name and constraints which govern the appearance of that
name in documents governed by the associated schema.
Elements are declared using the element element, and
attributes are declared using the attribute element. For
example, USAddress is defined as a complex
type, and within the definition of USAddress
we see five element declarations and one attribute
declaration:
| Defining the USAddress Type |
<xsd:complexType name="USAddress" >
<xsd:sequence>
<xsd:element name="name" type="xsd:string" />
<xsd:element name="street" type="xsd:string" />
<xsd:element name="city" type="xsd:string" />
<xsd:element name="state" type="xsd:string" />
<xsd:element name="zip" type="xsd:decimal" />
</xsd:sequence>
<xsd:attribute name="country" type="xsd:NMTOKEN"
use="fixed" value="US"/>
</xsd:complexType>
|
The consequence of this definition is
that any element appearing in an instance whose type is
declared to be USAddress (e.g.
shipTo in
po.xml) must consist of five elements and one
attribute. These elements must be called name,
street, city, state
and zip as specified by the values of the
declarations' name attributes, and the elements
must appear in the same sequence (order) in which they are
declared. The first four
of these elements will each contain a string, and the fifth
will contain a decimal number. The element whose type is
declared to be USAddress may appear with an
attribute called country which must contain
the string US.
The USAddress definition
contains only declarations involving simple types: string,
decimal and
NMTOKEN. In contrast, the
PurchaseOrderType definition contains element
declarations involving complex types, e.g.
USAddress, although note that both declarations use
the same type
attribute to identify the type, regardless of whether the
type is simple or complex.
| Defining PurchaseOrderType |
<xsd:complexType name="PurchaseOrderType"> <xsd:sequence> <xsd:element name="shipTo" type="USAddress" /> <xsd:element name="billTo" type="USAddress" /> <xsd:element ref="comment" minOccurs="0"/> <xsd:element name="items" type="Items" /> </xsd:sequence> <xsd:attribute name="orderDate" type="xsd:date" /> </xsd:complexType> |
In defining PurchaseOrderType, two of the
element declarations, for shipTo and
billTo, associate different element names with the
same complex type, namely USAddress. The
consequence of this definition is that any element
appearing in an instance (e.g.
po.xml) whose type is declared to be
PurchaseOrderType must consist of elements named
shipTo and billTo, each
containing the five subelements (name,
street, city, state and
zip) that were declared as part of
USAddress. The shipTo and
billTo elements may also carry the
country attribute that was declared as part of
USAddress.
The PurchaseOrderType definition contains an
orderDate attribute declaration which, like
the country attribute declaration, identifies
a simple type. In fact, all attribute declarations must
reference simple types because, unlike element
declarations, attributes cannot contain other elements or
other attributes.
The element declarations we have described so far have each associated a name with an existing type definition. Sometimes it is preferable to use an existing element rather than declare a new element, for example:
<xsd:element ref="comment" minOccurs="0"/>
This declaration references an existing element,
comment, that was declared elsewhere in the purchase
order schema. In general, the value of the ref attribute must reference a
global element, i.e. one that has been declared under
schema rather
than as part of a complex type definition. The consequence
of this declaration is that an element called
comment may appear in an instance document, and its
content must be consistent with that element's type, in
this case, string.
Both elements and attributes may be declared globally.
comment is one example of a global element
which we reference from an element declaration contained in
the PurchaseOrderType definition. We could
similarly declare attributes under schema, and reference them
using the ref
attribute from attribute declarations contained in type
definitions. Note that global declarations cannot contain
references, global declarations must identify simple and complex
types directly.
The comment element is
optional within PurchaseOrderType because the
value of the
minOccurs attribute in its declaration is 0. In
general, an element is required to appear when the value of
minOccurs
is 1 or more. The maximum number of times an element may
appear is determined by the value of a maxOccurs attribute in
its declaration. This may be a positive integer value such
as 41, or the term unbounded to indicate there
is no maximum number of occurrences. The default value for
both the
minOccurs and the maxOccurs attributes is
1. Thus, when an element such as comment is
declared without a
maxOccurs attribute, the element may not occur
more than once. Be sure that if you specify a value for
only the
minOccurs attribute, it is less than or equal to
the default value of
maxOccurs, i.e. it is 0 or 1. Similarly, if you
specify a value for only the maxOccurs attribute, it
must be greater than or equal to the default value of
minOccurs,
i.e. 1 or more. If both attributes are omitted, the element
must appear exactly once.
Attributes may appear once or not at
all (the default), and so the syntax for specifying
occurrences of attributes is different than the syntax for
elements. In particular, a
use attribute is used in an attribute
declaration to indicate whether the attribute is
required or optional, and if
optional whether the attribute's value is
fixed or whether there is a default. A
second attribute,
value, provides any value that is called for. To
illustrate, po.xsd
contains a declaration for the country
attribute, which is declared with use and value values of
fixed and US respectively. This
declaration means that the appearance of a
country attribute is optional, although its value
must be US if it does appear, and if it does
not appear, a schema processor will create a
country attribute with this value.
The values of the attributes used in element and attribute declarations to constrain the occurrences of elements and attributes are summarised in Table 1.
| Table 1. Occurrence Constraints for Elements and Attributes | ||
|---|---|---|
|
Elements (minOccurs, maxOccurs) fixed, default |
Attributes use, value |
Notes |
| (1, 1) -, - | required, - | element/attribute must appear once, it may have any value |
| (1, 1) 37, - | required, 37 | element/attribute must appear once, its value must be 37 |
| (2, unbounded) 37, - | n/a | element must appear twice or more, its value must be 37; in general, minOccurs and maxOccurs' values may be positive integers, and maxOccurs' value may also be "unbounded" |
| (0, 1) -, - | optional, - | element/attribute may appear once, it may have any value |
| (0, 1) 37, - | fixed, 37 | element/attribute may appear once, if it does appear its value must be 37, if it does not appear its value is 37 |
| (0, 1) -, 37 | default, 37 | element/attribute may appear once; if it does not appear its value is 37, otherwise its value is that given |
| (0, 2) -, 37 | n/a | element may appear once, twice, or not at all; if it does not appear its value is 37, otherwise its value is that given; in general, minOccurs and maxOccurs' values may be positive integers, and maxOccurs' value may also be "unbounded" |
| (0, 0) -, - | prohibited, - | element/attribute must not appear |
| Note that neither minOccurs, maxOccurs, nor use may appear in the declarations of global elements and attributes. | ||
So far, we have described how to define new complex types
(e.g. PurchaseOrderType), and declare elements
(e.g. purchaseOrder) and attributes (e.g.
orderDate). These activities generally involve
naming, and the question naturally arises: What happens if
two things are given the same name? The answer depends upon
the two things in question, although in general the more
similar are the two things, the more likely there will be
a conflict.
Here are some examples to illustrate when same names cause problems. If the two things are both types, say I define a complex type called USStates and a simple type called USStates, there is a conflict. If the two things are a type and an element or attribute, say I define a complex type called USAddress and I declare an element called USAddress, there is no conflict. If the two things are elements within different types (i.e. not global elements), say I declare one element called name as part of the USAddress type and a second element called name as part of the Item type, there is no conflict. (Such elements are sometimes called local element declarations). Finally, if the two things are both types and you define one and XML Schema has defined the other, say you define a simple type called decimal, there is no conflict. The reason for the apparent contradiction in the last example is that the two types belong to different namespaces. We'll explore the use of schema and namespaces in a later section.
The purchase order schema declares several elements and
attributes that have simple types. Some of these simple
types, such as string
and decimal, are
built-in to XML Schema, while others are derived from the
built-in's. For example, the partNum attribute
has a type called SKU (Stock Keeping Unit)
that is derived from
string. Both built-in simple types and their
derivations can be used in all element and attribute
declarations. Table 2 lists
all the simple types built-in to XML Schema, along with an
example of each type.
| Table 2. Simple Types Built-In to XML Schema | ||
|---|---|---|
| Simple Type | Examples (delimited by commas) | Notes |
| string | Confirm this is electric | |
| byte | -1, 126 | see (3) |
| unsignedByte | 0, 126 | see (3) |
| binary | 62696E617279 | see (1) |
| integer | -126789, -1, 0, 1, 126789 | see (3) |
| positiveInteger | 1, 126789 | see (3) |
| negativeInteger | -126789, -1 | see (3) |
| nonNegativeInteger | 0, 1, 126789 | see (3) |
| nonPositiveInteger | -126789, -1, 0 | see (3) |
| int | -1, 126789675 | see (3) |
| unsignedInt | 0, 1267896754 | see (3) |
| long | -1, 12678967543233 | see (3) |
| unsignedLong | 0, 12678967543233 | see (3) |
| short | -1, 12678 | see (3) |
| unsignedShort | 0, 12678 | see (3) |
| decimal | -1.23, 0, 123.4, 1000.00 | see (3) |
| float | -INF, -1E4, -0, 0, 12.78E-2, 12, INF, NaN | equivalent to single-precision 32-bit floating point, NaN is "not a number", see (3) |
| double | -INF, -1E4, -0, 0, 12.78E-2, 12, INF, NaN | equivalent to double-precision 64-bit floating point, see (3) |
| boolean | true, false | |
| time | 13:20:00.000, 13:20:00.000-05:00 | see (3) |
| timeInstant | 1999-05-31T13:20:00.000-05:00 | May 31st 1999 at 1.20pm Eastern Standard Time which is 5 hours behind Co-Ordinated Universal Time, see (3) |
| timePeriod | 1999-05-31T13:20 | see (3) |
| timeDuration | P1Y2M3DT10H30M12.3S | 1 year, 2 months, 3 days, 10 hours, 30 minutes, 12.3 seconds |
| date | 1999-05-31 | see (3) |
| month | 1999-05 | May 1999, see (3) |
| year | 1999 | 1999, see (3) |
| century | 19 | the 1900's, see (3) |
| recurringDay | ----31 | every 31st day, see (3) |
| recurringDate | --05-31 | every May 31st, see (3) |
| recurringDuration | --05-31T13:20:00 | May 31st every year at 1.20pm Co-Ordinated Universal Time, format similar to timeInstant, see (1) & (3) |
| Name | shipTo | XML 1.0 Name type |
| QName | po:USAddress | XML Namespace QName |
| NCName | USAddress | XML Namespace NCName, i.e. a QName without the prefix and colon |
| uriReference | http://www.example.com/, http://www.example.com/doc.html#ID5 | |
| language | en-GB, en-US, fr | valid values for xml:lang as defined in XML 1.0 |
| ID | XML 1.0 ID attribute type, see (2) | |
| IDREF | XML 1.0 IDREF attribute type, see (2) | |
| IDREFS | XML 1.0 IDREFS attribute type, see (2) | |
| ENTITY | XML 1.0 ENTITY attribute type, see (2) | |
| ENTITIES | XML 1.0 ENTITIES attribute type, see (2) | |
| NOTATION | XML 1.0 NOTATION attribute type, see (2) | |
| NMTOKEN | US, Brésil | XML 1.0 NMTOKEN attribute type, see (2) |
| NMTOKENS | US UK, Brésil Canada Mexique | XML 1.0 NMTOKENS attribute type, i.e. a whitespace separated list of NMTOKEN's, see (2) |
| Notes: (1) Authors must apply facets to the simple types binary and recurringDuration types in order to use them. (2) To retain compatibility between XML Schema and XML 1.0 DTDs, the simple types ID, IDREF, IDREFS, ENTITY, ENTITIES, NOTATION, NMTOKEN, NMTOKENS should only be used in attributes. (3) A value of this type can be represented by more than one lexical format, e.g. 100 and 1.0E2 are both valid float formats representing "one hundred". However, rules have been established for this type that define a canonical lexical format, see XML Schema Part 2. | ||
New simple types are defined by
deriving them from existing simple types (built-in's and
derived). In particular, we can derive a new simple type by
restricting an existing simple type, in other words, the
legal range of values for the new type are a subset of the
existing type's range of values. We use the simpleType element to
define and name the new simple type. We use the restriction element to
indicate the existing (base) type, and to identify the
"facets" that constrain the range of values. A complete
list of facets is provided in
Appendix B.
Suppose we wish to create a new type
of integer called myInteger whose range of
values is between 10000 and 99999 (inclusive). We base our
definition on the built-in simple type integer, whose range of values also
includes integers less than 10000 and greater than 99999. To
define myInteger, we restrict the range of the
integer base type by
employing two facets, minInclusive and
maxInclusive:
| Defining myInteger, Range 10000-99999 |
<xsd:simpleType name="myInteger">
<xsd:restriction base="xsd:integer">
<xsd:minInclusive value="10000"/>
<xsd:maxInclusive value="99999"/>
</xsd:restriction>
</xsd:simpleType>
|
The example shows one particular combination of a base
type and two facets used to define myInteger, but
a look at the list of built-in simple types and their
facets (Appendix B) should
suggest other viable combinations.
The purchase order schema contains
another, more elaborate, example of a simple type
definition. A new simple type called SKU is
derived (by restriction) from the simple type
string. Furthermore, we constrain the values of
SKU using a facet called pattern in conjunction with
the regular expression "\d{3}-[A-Z]{2}" that
is read "three digits followed by a hyphen followed by two
upper-case ASCII letters":
| Defining the Simple Type "SKU" |
<xsd:simpleType name="SKU">
<xsd:restriction base="xsd:string">
<xsd:pattern value="\d{3}-[A-Z]{2}"/>
</xsd:restriction>
</xsd:simpleType>
|
This regular expression language is described more fully in Appendix C.
XML Schema defines fourteen facets
which are listed in
Appendix B. Among these, the enumeration facet is one
the most useful and it can be used to constrain the values
of almost every simple type, except the boolean type. The enumeration facet limits
a simple type to a set of distinct values. For example, we
can use the
enumeration facet to define a new simple type
called USState, derived from string, whose value must be one of the
standard US state abbreviations:
| Using the Enumeration Facet |
<xsd:simpleType name="USState">
<xsd:restriction base="xsd:string">
<xsd:enumeration value="AK"/>
<xsd:enumeration value="AL"/>
<xsd:enumeration value="AR"/>
<!-- and so on ... -->
</xsd:restriction>
</xsd:simpleType>
|
USState would be a good replacement for the
string type currently
used in the state element declaration. By
making this replacement, the legal values of a
state element, i.e. the state
subelements of billTo and
shipTo, would be limited to one of AK,
AL, AR, etc. Note that the
enumeration values specified for a particular type must be
unique.
XML Schema has the concept of a list type, in addition to the so-called
atomic types that constitute most of the types listed in Table 2. The value
of an atomic type is indivisible from XML Schema's perspective; For
example, the NMTOKEN value US
is indivisible in the sense that no part of US, such as the
character "S", has any meaning by itself.
In contrast, list types are comprised of sequences of atomic types and
consequently the
parts of a sequence (the "atoms") themselves are meaningful. For
example, NMTOKENS is a
list type, and an element of this type would be a
white-space delimited list of
NMTOKEN's, such as "US UK FR". XML Schema has three
built-in list types, they are
NMTOKENS,
IDREFS, and
ENTITIES.
In addition to using the built-in
list types, you can create new list types by derivation
from existing atomic types. (You cannot create list types
from existing list types, nor from complex types). For
example, to create a list of myInteger's:
| Creating a List of myInteger's |
<xsd:simpleType name="listOfMyIntType"> <xsd:list itemType="myInteger"/> </xsd:simpleType> |
And an element in an instance document whose content
conforms to listOfMyIntType is:
<listOfMyInt>20003 15037 95977 95945</listOfMyInt>
Several facets can be applied to list types:
length, minLength, maxLength, and enumeration. For
example, to define a list of exactly six US states (SixUSStates), we first
define a new list type called USStateList from
USState, and then we derive SixUSStates
by restricting USStateList to only six items:
| List Type for Six US States |
<xsd:simpleType name="USStateList"> <xsd:list itemType="USState"/> </xsd:simpleType> <xsd:simpleType name="SixUSStates"> <xsd:restriction base="USStateList"> <xsd:length value="6"/> </xsd:restriction> </xsd:simpleType> |
Elements whose type is SixUSStates must have six items,
and each of the six items must be one of the (atomic) values of
the enumerated type USState, for example:
<sixStates>PA NY CA NY LA AK</sixStates>
Note that it is possible to derive a list type from the
atomic type string.
However, a string may
contain white space, and white space delimits the items in
a list type, so you should be careful using fixed length
list types whose base type is
string. For example, suppose a list type is
defined with a
length facet equal to 3, and base type string, then the following 3 item
list is legal:
Asie Europe Afrique
But the following 3 "item" list is illegal:
Asie Europe Amérique Latine
Even though "Amérique Latine" may exist as a single string outside of the list, when it is included in the list, the whitespace between Amérique and Latine effectively creates a fourth item, and so the latter example will not conform to the 3-item list type.
Atomic types and list types enable an element or an attribute value to
be one or more instances of one atomic type. In contrast,
a union type enables an element or attribute value to be one
or more instances of one type drawn from the union of multiple atomic
and list types. To illustrate, we create
a union type for representing American states as singleton letter abbreviations
or lists of numeric codes. The
zipList union type is built from one atomic type and one list
type:
| Union Type for Zipcodes |
<xsd:simpleType name="zipUnion"> <xsd:union memberTypes="USState listOfMyIntType"/> </xsd:simpleType> |
When we define a union type, the memberTypes attribute
value is a list of all the types in the union.
Now, assuming we have declared an element called zips of type
zipUnion, valid instances of the element are:
<zips>CA</zips>
<zips>95630 95977 95945</zips>
<zips>AK</zips>
Two facets, pattern and
enumeration, can be
applied to a union type.
Schemas can be constructed by defining sets of named types
such as PurchaseOrderType and then declaring
elements such as purchaseOrder that reference
the types using the
type= construction. This style of schema
construction is straightforward but it can be unwieldy,
especially if you define many types that are referenced
only once and contain very few constraints. In these cases,
a type can be more succinctly defined as an anonymous type
which saves the overhead of having to be named and
explicitly referenced.
The definition of the type Items in po.xsd contains two element
declarations that use anonymous types (item
and quantity). In general, you can identify
anonymous types by the lack of a type= in an element (or
attribute) declaration, and by the presence of an
un-named (simple or complex) type definition:
| Two Anonymous Type Definitions |
<xsd:complexType name="Items">
<xsd:sequence>
<xsd:element name="item" minOccurs="0" maxOccurs="unbounded">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="productName" type="xsd:string"/>
<xsd:element name="quantity">
<xsd:simpleType>
<xsd:restriction base="xsd:positiveInteger">
<xsd:maxExclusive value="100"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:element>
<xsd:element name="USPrice" type="xsd:decimal"/>
<xsd:element ref="comment" minOccurs="0"/>
<xsd:element name="shipDate" type="xsd:date" minOccurs="0"/>
</xsd:sequence>
<xsd:attribute name="partNum" type="SKU"/>
</xsd:complexType>
</xsd:element>
</xsd:sequence>
</xsd:complexType>
|
In the case of the item element, it has an
anonymous complex type consisting of the elements
productName, quantity,
USPrice, comment, and
shipDate, and an attribute called
partNum. In the case of the quantity
element, it has an anonymous simple type derived from
integer whose value
ranges between 1 and 99.
The purchase order schema has many examples of elements
containing other elements (e.g. items),
elements having attributes and containing other elements
(e.g. shipTo), and elements containing only a
simple type of value (e.g. USPrice). However,
we have not seen an element having attributes but
containing only a simple type of value, nor have we seen an
element that contains other elements mixed with character
content, nor have we seen an element that has no content at
all. In this section we'll examine these variations in the
content models of elements.
Let us first consider how to declare an element that has an attribute and contains a simple value. In an instance document, such an element might appear as:
<internationalPrice currency="EUR">423.46</internationalPrice>
The purchase order schema declares a USPrice
element that is a starting point:
<xsd:element name="USPrice" type="decimal"/>
Now, how do we add an attribute to
this element? As we have said before, simple types cannot
have attributes, and
decimal is a simple type. Therefore, we must
define a complex type to carry the attribute declaration.
We also want the content to be simple type decimal. So our original question
becomes: How do we define a complex type that is based on
the simple type
decimal? The answer is to derive a new
complex type from the simple type
decimal:
| Deriving a Complex Type from a Simple Type |
<xsd:element name="internationalPrice">
<xsd:complexType>
<xsd:simpleContent>
<xsd:extension base="xsd:decimal">
<xsd:attribute name="currency" type="xsd:string" />
</xsd:extension>
</xsd:simpleContent>
</xsd:complexType>
</xsd:element>
|
We use the
complexType element to start the definition of a new
(anonymous) type.
To indicate that the content model of
the new type contains only character data and no
elements, we use a
simpleContent
element. Finally, we derive the new type by extending the simple
decimal type. The extension
consists of adding a currency attribute using a
standard attribute declaration. (We cover type derivation
in detail in Section 4). The
internationalPrice element declared in this way will
appear in an instance as shown in the example above.
The construction of the purchase order schema may be characterized as elements containing subelements, and the deepest subelements contain character data. XML Schema also provides for the construction of schemas where character data can appear alongside subelements, and character data is not confined to the deepest subelements.
To illustrate, consider the following snippet from a customer letter that uses some of the same elements as the purchase order:
| Snippet of Customer Letter |
<letterBody> <salutation>Dear Mr.<name>Robert Smith</name>.</salutation> Your order of <quantity>1</quantity> <productName>Baby Monitor</productName> shipped from our warehouse on <shipDate>1999-05-21</shipDate>. .... </letterBody> |
Notice the text appearing between elements and their child
elements. Specifically, text appears between the elements
salutation, quantity,
productName and shipDate which are all
children of letterBody, and text appears
around the element name which is the child of a child of
letterBody. The following snippet of a schema
declares letterBody:
| Snippet of Schema for Customer Letter |
<xsd:element name="letterBody">
<xsd:complexType mixed="true">
<xsd:sequence>
<xsd:element name="salutation">
<xsd:complexType mixed="true">
<xsd:sequence>
<xsd:element name="name" type="xsd:string"/>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
<xsd:element name="quantity" type="xsd:positiveInteger"/>
<xsd:element name="productName" type="xsd:string"/>
<xsd:element name="shipDate" type="xsd:date" minOccurs="0"/>
<!-- etc -->
</xsd:sequence>
</xsd:complexType>
</xsd:element>
|
The elements appearing in the customer letter are declared, and
their types are defined using the element
and complexType element
constructions we have seen before. To enable character data to appear
between the child-elements of letterBody, the
mixed attribute on the
type definition is set to true.
Note that the mixed model in XML Schema
differs fundamentally from the
mixed model in XML 1.0. Under the XML Schema
mixed model, the order and number of child elements
appearing in an instance must agree with the order and
number of child elements specified in the model. In
contrast, under the XML 1.0 mixed model, the order and
number of child elements appearing in an instance cannot be
constrained. In sum, XML Schema provides full schema
validation of mixed models in contrast to the partial
schema validation provided by XML 1.0.
Now suppose that we want the
internationalPrice element to convey both the unit
of currency and the price as attribute values rather than
as separate attribute and content values. For example:
<internationalPrice currency="EUR" value="423.46" />
Such an element has no content at all, and we say that its content model is empty. To define a type whose content is empty, we essentially define a type that allows only elements in its content, but we do not actually declare any elements and so the type's content model is empty:
| An Empty Complex Type |
<xsd:element name="internationalPrice">
<xsd:complexType>
<xsd:complexContent>
<xsd:restriction base="xsd:anyType">
<xsd:attribute name="currency" type="xsd:string"/>
<xsd:attribute name="value" type="xsd:decimal"/>
</xsd:restriction>
</xsd:complexContent>
</xsd:complexType>
</xsd:element>
|
In this example, we define an (anonymous) type having complexContent,
i.e. only elements.
The complexContent element signals that we intend to restrict or
extend the content model of a complex type; The restriction of
anyType declares two attributes but does not introduce any
element content (see
Section 4.4 for more details on restriction).
The internationalPrice element declared in this way will
appear in an instance as shown in the example above.
The anyType represents an abstraction called the
ur-type which is the base type from which all simple
and complex types are derived. An anyType type does
not constrain its content in any way. It is possible to use
anyType like other types, for example:
<xsd:element name="anything" type="xsd:anyType" />
The content of the element declared in this way is
unconstrained, so the element value may be 423.46, but
it may be any other sequence of characters as
well.
In general, it is probably better to avoid unconstrained
types in favour of constrained types such as decimal,
string, etc.
XML Schema provides three elements
for annotating schemas for the benefit of both human
readers and applications. In the purchase order schema, we
put a basic schema description and copyright information
inside the
documentation element, which is the recommended
location for human readable material.
The
appInfo element, which we did not use in the
purchase order schema, can be used to provide information
for tools, stylesheets and other applications. An
interesting example using
appInfo is one of the schemas
that describes some of the simple types in XML Schema Part
2: Datatypes. Information describing this schema, e.g.
which facets are applicable to particular simple types, is
represented inside
appInfo elements, which was used by an
application to automatically generate text for the XML
Schema Part 2 document.
Both documentation and
appInfo appear
as subelements of
annotation, which may itself appear at the
beginning of most schema constructions. To illustrate, the
following example shows annotation elements
appearing at the beginning of an element declaration and a
complex type definition:
| Annotations in Element Declaration & Complex Type Definition |
<xsd:element name="internationalPrice">
<xsd:annotation>
<xsd:documentation>element declared with anonymous type</xsd:documentation>
</xsd:annotation>
<xsd:complexType>
<xsd:annotation>
<xsd:documentation>empty anonymous type with 2 attributes</xsd:documentation>
</xsd:annotation>
<xsd:complexContent>
<xsd:restriction base="xsd:anyType">
<xsd:attribute name="currency" type="xsd:string" />
<xsd:attribute name="value" type="xsd:decimal" />
</xsd:restriction>
</xsd:complexContent>
</xsd:complexType>
</xsd:element>
|
The
annotation element may also appear at the
beginning of other schema constructions such as those
indicated by the elements
schema,
simpleType, and attribute.
The definitions of complex types in the purchase order
schema all declare sequences of elements that must appear
in the instance document. The occurrence of individual
elements declared in the so-called content models of these
types may be optional, as indicated by a 0 value for the
attribute
minOccurs (e.g. in comment), or
otherwise constrained depending upon the values of minOccurs and
maxOccurs. XML
Schema also provides constraints that apply to groups of
elements appearing in a content model. These constraints
mirror those available in XML 1.0 plus some additional
constraints. Note that the
constraints do not apply to attributes.
XML Schema enables a group of elements to be defined and named, so that the elements can be used to build up the content models of complex types (thus mimicking common usage of parameter entities in XML 1.0). Un-named groups of elements can also be defined, and along with elements in named groups, they can be constrained to appear in the same order (sequence) as they are declared. Alternatively, they can be constrained so that only one of the elements may appear in an instance.
To illustrate, we modify the
PurchaseOrderType definition from the purchase order schema
using two groups so that purchase orders may contain either
separate shipping and billing addresses, or a single
address for those cases in which the shippee and billee are
co-located:
| Nested Choice and Sequence Groups |
<xsd:complexType name="PurchaseOrderType">
<xsd:sequence>
<xsd:choice>
<xsd:group ref="shipAndBill" />
<xsd:element name="singleUSAddress" type="USAddress" />
</xsd:choice>
<xsd:element ref="comment" minOccurs="0"/>
<xsd:element name="items" type="Items" />
</xsd:sequence>
<xsd:attribute name="orderDate" type="xsd:date" />
</xsd:complexType>
<xsd:group name="shipAndBill">
<xsd:sequence>
<xsd:element name="shipTo" type="USAddress" />
<xsd:element name="billTo" type="USAddress" />
</xsd:sequence>
</xsd:group>
|
The choice group
element allows only one of its children to appear in an
instance. One child is an inner group element that references
the named group shipAndBill consisting of the
element sequence shipTo, billTo,
and the second child is a singleUSAddress.
Hence, in an instance document, the
purchaseOrder element must contain either a shipTo
element followed by a billTo element or
a singleUSAddress element. The
choice group is followed
by the comment and items element declarations,
and both the choice group
and the element declarations are children of a
sequence group.
The effect of these various groups is that the address element(s) must be
followed by comment and items elements in that
order.
There exists a third option for
constraining elements in a group: All the elements in the
group may appear once or not at all, and they may appear in
any order. The all
group (which provides a simplified version of the SGML
&-Connector) is limited to the top-level of any content
model. Moreover, the group's children must all be
individual elements (no groups), and no element in the
content model may appear more than once, i.e. the
permissible values of
minOccurs and maxOccurs are 0 and 1.
For example, to allow the child elements of
purchaseOrder to appear in any order, we could
redefine PurchaseOrderType as:
| An 'All' Group |
<xsd:complexType name="PurchaseOrderType">
<xsd:all>
<xsd:element name="shipTo" type="USAddress"/>
<xsd:element name="billTo" type="USAddress"/>
<xsd:element ref="comment" minOccurs="0"/>
<xsd:element name="items" type="Items" />
</xsd:all>
<xsd:attribute name="orderDate" type="xsd:date" />
</xsd:complexType>
|
By this definition, a comment element may
optionally appear within purchaseOrder, and it
may appear before or after any shipTo,
billTo and items elements, but it can
appear only once. Moreover, the stipulations of an all group do not allow us to
declare an element such as comment outside the
group as a means of enabling it to appear more than once.
XML Schema stipulates that an
all group must appear as the sole child at the
top of a content model. In other words, the following is
illegal:
| Illegal Example with an 'All' Group |
<xsd:complexType name="PurchaseOrderType">
<xsd:sequence>
<xsd:all>
<xsd:element name="shipTo" type="USAddress"/>
<xsd:element name="billTo" type="USAddress"/>
<xsd:element name="items" type="Items" />
</xsd:all>
<xsd:sequence>
<xsd:element ref="comment" minOccurs="0" maxOccurs="unbounded"/>
</xsd:sequence>
</xsd:sequence>
<xsd:attribute name="orderDate" type="xsd:date"/>
</xsd:complexType>
|
Finally, named and un-named groups that appear in content
models (represented by
group and
choice,
sequence,
all respectively) may carry minOccurs and maxOccurs
attributes. By combining and nesting the various groups
provided by XML Schema, and by setting the values of
minOccurs and
maxOccurs,
it is possible to represent any content model expressible
with an XML 1.0 DTD. Furthermore, the all group provides additional
expressive power.
Suppose we want to provide more information about each
item in a purchase order, for example, each item's weight
and preferred shipping method. We can accomplish this by
adding weightKg and shipBy
attribute declarations to the item element's
(anonymous) type definition:
| Adding Attributes to the Inline Type Definition |
<xsd:element name="Item" minOccurs="0" maxOccurs="unbounded">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="productName" type="xsd:string"/>
<xsd:element name="quantity">
<xsd:simpleType>
<xsd:restriction base="xsd:positiveInteger">
<xsd:maxExclusive value="100"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:element>
<xsd:element name="USPrice" type="xsd:decimal"/>
<xsd:element ref="comment" minOccurs="0"/>
<xsd:element name="shipDate" type="xsd:date" minOccurs="0"/>
</xsd:sequence>
<xsd:attribute name="partNum" type="SKU"/>
<!-- add weightKg and shipBy attributes -->
<xsd:attribute name="weightKg" type="xsd:decimal"/>
<xsd:attribute name="shipBy">
<xsd:simpleType>
<xsd:restriction base="xsd:string">
<xsd:enumeration value="air"/>
<xsd:enumeration value="land"/>
<xsd:enumeration value="any"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:attribute>
</xsd:complexType>
</xsd:element>
|
Alternatively, we can create a named
attribute group containing all the desired attributes of an
item element, and reference this group by name
in the item element declaration:
| Adding Attributes Using an Attribute Group |
<xsd:element name="item" minOccurs="0" maxOccurs="unbounded">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="productName" type="xsd:string"/>
<xsd:element name="quantity">
<xsd:simpleType>
<xsd:restriction base="xsd:positiveInteger">
<xsd:maxExclusive value="100"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:element>
<xsd:element name="USPrice" type="xsd:decimal"/>
<xsd:element ref="comment" minOccurs="0"/>
<xsd:element name="shipDate" type="xsd:date" minOccurs="0"/>
</xsd:sequence>
<!-- attributeGroup replaces individual declarations -->
<xsd:attributeGroup ref="ItemDelivery"/>
</xsd:complexType>
</xsd:element>
<xsd:attributeGroup name="ItemDelivery">
<xsd:attribute name="partNum" type="SKU"/>
<xsd:attribute name="weightKg" type="xsd:decimal"/>
<xsd:attribute name="shipBy">
<xsd:simpleType>
<xsd:restriction base="xsd:string">
<xsd:enumeration value="air"/>
<xsd:enumeration value="land"/>
<xsd:enumeration value="any"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:attribute>
</xsd:attributeGroup>
|
Using an attribute group in this way can improve the readability of schema, and facilitates updating schema because an attribute group can be defined and edited in one place and referenced in multiple definitions and declarations. These characteristics of attribute groups make them similar to parameter entities in XML 1.0. Note that an attribute group may contain other attribute groups. Note also that both attribute declarations and attribute group references must appear at the end of complex type definitions.
One of the purchase order items listed in po.xml, the Lawnmower,
does not have a shipDate element. Within the
context of our scenario, the schema author may have
intended such absences to indicate items not
yet shipped. But in general, the absence of an element does
not have any particular meaning: It may indicate that the
information is unknown, or not applicable, or the element
may be absent for some other reason. Sometimes it is
desirable to represent an unshipped item,
unknown information, or inapplicable information
explicitly with an element, rather than by an absent
element. For example, it may be desirable to represent a
"null" value being sent to or from a relational database
with an element that is present. Such cases can be
represented using XML Schema's null mechanism which enables
an element to appear with or without a non-null value.
XML Schema's null mechanism involves
an "out of band" null signal. In other words, there is no
actual null value that appears as element content, instead
there is an attribute to indicate that the element content
is null. To illustrate, we can modify the
shipDate element declaration so that nulls can be
signalled:
<xsd:element name="shipDate" type="xsd:date" nullable="true"/>
And to explictly represent that
shipDate has a null value in the instance
document, we set the null attribute (from the XML Schema
namespace for instances) to true:
<shipDate xsi:null="true"></shipDate>
The null
attribute is defined as part of the XML Schema namespace
for instances
(http://www.w3.org/1999/XMLSchema-instance),
and so it must appear in the instance document with a
prefix (xsi:) associated with that namespace.
(As with the xsd: prefix, the
xsi: prefix is used by convention only). Note that
the null mechanism applies only to element values, and not
to attribute values. An element with xsi:null="true" may not
have any element content but it may still carry attributes.
A schema can be viewed as a
collection (vocabulary) of type definitions and element
declarations whose names belong to a particular namespace
called a target namespace. The target namespace enables us
to distinguish between definitions and declarations from
different vocabularies. For example, target namespaces
would enable us to distinguish between the declaration for
element in the
XML Schema language vocabulary, and a declaration for
element in a hypothetical chemistry language
vocabulary. The former is part of the
http://www.w3.org/2000/08/XMLSchema target namespace,
and the latter is part of another target namespace.
When we want to check that an instance document conforms to one or more schemas (through a process called schema validation), we need to identify which element and attribute declarations and type definitions in the schemas should be used to check which elements and attributes in the instance document. The target namespace plays an important role in the identification process. We examine the role of the target namespace in the next section.
The schema author also has several options that affect how the identities of elements and attributes are represented in instance documents. More specifically, the author can decide whether or not the appearance of locally declared elements and attributes in an instance must be qualified by a namespace, using either an explicit prefix or implicitly by default. The schema author's choice regarding qualification of local elements and attributes has a number of implications regarding the structures of schemas and instance documents, and we examine some of these implications in the following sections.
In a new version of the purchase order schema (po1.xsd), we explicitly declare
a target namespace, and specify that both locally defined
elements and locally defined attributes must be
unqualified. The target namespace in po1.xsd is
http://www.example.com/PO1, as indicated by the
value of the
targetNamespace attribute.
Qualification of local elements and attributes can be
globally specified by a pair of attributes,
elementFormDefault and
attributeFormDefault, on the schema element, or can be
specified separately for each local declaration using the
form attribute.
All such attributes' values may each be set to
unqualified or qualified, to indicate
whether or not locally declared elements and attributes
must be unqualified.
In po1.xsd we globally
specify the qualification of elements and attributes by
setting the values of both
elementFormDefault and
attributeFormDefault to
unqualified. Strictly speaking, these settings are
unnecessary because the values are the defaults for the two
attributes; We make them to highlight the contrast between
this case and other cases to be described later.
| Purchase Order Schema with Target Namespace, po1.xsd |
<schema xmlns="http://www.w3.org/2000/08/XMLSchema"
xmlns:po="http://www.example.com/PO1"
targetNamespace="http://www.example.com/PO1"
elementFormDefault="unqualified"
attributeFormDefault="unqualified">
<element name="purchaseOrder" type="po:PurchaseOrderType"/>
<element name="comment" type="string"/>
<complexType name="PurchaseOrderType">
<sequence>
<element name="shipTo" type="po:USAddress"/>
<element name="billTo" type="po:USAddress"/>
<element ref="po:comment" minOccurs="0"/>
<!-- etc -->
</sequence>
</complexType>
<complexType name="USAddress">
<sequence>
<element name="name" type="string"/>
<element name="street" type="string"/>
<!-- etc -->
</sequence>
</complexType>
<!-- etc -->
</schema>
|
To see how the target namespace of this schema is
populated, we'll examine in turn each of the type
definitions and element declarations. Starting from the end
of the schema, we first define a type called
USAddress that consists of the elements
name, street, etc. One consequence of
this type definition is that the USAddress
type is included in the schema's target namespace. We next
define a type called PurchaseOrderType that
consists of the elements shipTo,
billTo, comment, etc.
PurchaseOrderType is also included in the schema's
target namespace. Notice that the type references in the
three element declarations are prefixed, i.e.
po:USAddress, po:USAddress and
po:comment, and the prefix is associated with the
namespace http://www.example.com/PO1. This is
the same namespace as the schema's target namespace, and so
a processor of this schema will know to look within this
schema for the definition of the type
USAddress and the declaration of the element
comment. It is also possible to refer to types in
another schema with a different target namespace, hence
enabling re-use of definitions and declarations between
schemas.
At the beginning of the schema
po1.xsd, we declare the elements
purchaseOrder and comment. They are
included in the schema's target namespace. The
purchaseOrder element's type is prefixed, for the
same reason that USAddress is prefixed. In
contrast, the comment element's type, string, is not prefixed. The
po1.xsd schema
contains a default namespace declaration and so unprefixed
types such as string,
and unprefixed elements such as element and complexType, are
associated with the default namespace,
http://www.w3.org/2000/08/XMLSchema. In fact, this is
the target namespace of XML Schema itself, and so a
processor of po1.xsd
will know to look within the schema of XML Schema
(otherwise known as the "schema for schemas") for the
definition of the type
string and the declaration of the element called
element.
Let us now examine how the target namespace of the schema affects a conforming instance document:
The instance document declares one namespace,
http://www.example.com/PO1, and associates it with
the prefix apo:. This prefix is used to
qualify two elements in the document, namely
purchaseOrder and comment. The
namespace is the same as the target namespace of the schema
in po1.xsd, and so a
processor of the instance document will know to look in
that schema for the declarations of
purchaseOrder and comment. In fact,
target namespaces are so named because of the sense in
which there exists a target namespace for the elements
purchaseOrder and comment. Target
namespaces in the schema therefore control the validation
of corresponding namespaces in the instance.
The prefix apo: is applied to the global
elements purchaseOrder and
comment elements. Furthermore,
elementFormDefault and
attributeFormDefault require that the prefix is
not applied to any of the the locally declared
elements such as shipTo, billTo,
name and street, and it is
not applied to any of the attributes (which were all
declared locally). The purchaseOrder and
comment are global elements because they are
declared in the context of the schema as a whole rather
than within the context of a particular type. For example,
the declaration of purchaseOrder appears as a
child of the
schema element in
po1.xsd, whereas the declaration of
shipTo appears as a child of the complexType element that
defines PurchaseOrderType.
When local elements and attributes are not required to be
qualified, an instance author may require more or less
knowledge about the details of the schema to create schema
valid instance documents. More specifically, if the author
can be sure that only the root element (such as
purchaseOrder) is global, then it is a simple matter
to qualify only the root element. Alternatively, the author
may know that all the elements are declared globally, and
so all the elements in the instance document can be
prefixed, perhaps taking advantage of a default namespace
declaration. (We examine this approach in Section 3.3). On the other hand, if
there is no uniform pattern of global and local
declarations, the author will need detailed knowledge of
the schema to correctly prefix global elements (and
attributes).
Elements and attributes can be independently required to
be qualified, although we'll start by describing
qualification of local elements. To specify that all
locally declared elements in a schema must be qualified, we
set the value of
elementFormDefault to qualified:
| Modifications to po1.xsd for Qualified Locals |
<schema xmlns="http://www.w3.org/2000/08/XMLSchema"
xmlns:po="http://www.example.com/PO1"
targetNamespace="http://www.example.com/PO1"
elementFormDefault="qualified"
attributeFormDefault="unqualified">
<element name="purchaseOrder" type="po:PurchaseOrderType"/>
<element name="comment" type="string"/>
<complexType name="PurchaseOrderType">
<!-- etc -->
</complexType>
<!-- etc -->
</schema>
|
And in this conforming instance document, we qualify all the elements explicitly:
| A Purchase Order with Explicitly Qualified Locals |
<?xml version="1.0"?>
<apo:purchaseOrder xmlns:apo="http://www.example.com/PO1"
orderDate="1999-10-20">
<apo:shipTo country="US">
<apo:name>Alice Smith</apo:name>
<apo:street>123 Maple Street</apo:street>
<!-- etc -->
</apo:shipTo>
<apo:billTo country="US">
<apo:name>Robert Smith</apo:name>
<apo:street>8 Oak Avenue</apo:street>
<!-- etc -->
</apo:billTo>
<apo:comment>Hurry, my lawn is going wild!</apo:comment>
<!-- etc -->
</apo:purchaseOrder>
|
Alternatively, we can replace the explicit qualification of every element with implicit qualification provided by a default namespace, as shown here in po2.xml:
In po2.xml, all the elements in the instance belong to the same namespace, and the namespace statement declares a default namespace that applies to all the elements in the instance. Hence, it is unnecessary to explicitly prefix any of the elements. As another illustration of using qualified elements, the schemas in Section 5 all require qualified elements.
Qualification of attributes is very similar to the
qualification of elements. Attributes that must be
qualified, either because they are declared globally or
because the
attributeFormDefault attribute is set to
qualified, appear prefixed in instance documents.
One example of a qualified attribute is the xsi:null attribute that
was introduced in Section 2.9. In
fact, attributes that are required to be qualified must be
explicitly prefixed because the XML-Namespaces
specification does not provide a mechanism for defaulting
the namespaces of attributes. Attributes that are not
required to be qualified appear in instance documents
without prefixes, which is the typical case.
The qualification mechanism we have
described so far has controlled all local element and
attribute declarations within a particular target
namespace. It is also possible to control qualification on
a declaration by declaration basis using the form attribute. For example,
to require that the locally declared attribute
publicKey is qualified in instances, we declare it
in the following way:
| Requiring Qualification of Single Attribute |
<schema xmlns="http://www.w3.org/2000/08/XMLSchema"
xmlns:po="http://www.example.com/PO1"
targetNamespace="http://www.example.com/PO1"
elementFormDefault="qualified"
attributeFormDefault="unqualified">
<!-- etc -->
<element name="secure">
<complexType>
<sequence>
<!-- element declarations -->
</sequence>
<attribute name="publicKey" form="qualified">
<simpleType>
<restriction base="binary">
<encoding value="base64"/>
</restriction>
</simpleType>
</attribute>
</complexType>
</element>
</schema>
|
Notice that the value of the form attribute overides the
value of the
attributeFormDefault attribute for the
publicKey attribute only. Also, the form attribute can be applied
to an element declaration in the same manner. An instance
document that conforms to the schema is:
| Instance with a Qualified Attribute |
<?xml version="1.0"?>
<purchaseOrder xmlns="http://www.example.com/PO1"
xmlns:po="http://www.example.com/PO1"
orderDate="1999-10-20">
<!-- etc -->
<secure po:publicKey="GpM7">
<!-- etc -->
</secure>
</purchaseOrder>
|
Another authoring style, when all the element names are
unique within a namespace, is to create a schema in which
all elements are global. This is similar in effect to the
use of <!ELEMENT> in a DTD. In the example below, we
have modified the original po1.xsd such that all
the elements are declared globally. Notice that we have
omitted the
elementFormDefault and
attributeFormDefault attributes in this example
to emphasise that their values are irrelevant when there
are only global element and attribute declarations.
| Modified version of po1.xsd using only global element declarations |
<schema xmlns="http://www.w3.org/2000/08/XMLSchema"
xmlns:po="http://www.example.com/PO1"
targetNamespace="http://www.example.com/PO1">
<element name="purchaseOrder" type="po:PurchaseOrderType"/>
<element name="shipTo" type="po:USAddress"/>
<element name="billTo" type="po:USAddress"/>
<element name="comment" type="string"/>
<element name="name" type="string"/>
<element name="street" type="string"/>
<complexType name="PurchaseOrderType">
<sequence>
<element ref="po:shipTo"/>
<element ref="po:billTo"/>
<element ref="po:comment" minOccurs="0"/>
<!-- etc -->
</sequence>
</complexType>
<complexType name="USAddress">
<sequence>
<element ref="po:name"/>
<element ref="po:street"/>
<!-- etc -->
</sequence>
</complexType>
<!-- etc -->
</schema>
|
This "global" version of po1.xsd will validate the instance document po2.xml which, as we described previously, is also schema valid against the "qualified" version of po1.xsd. In other words, both schema approaches can validate the same, namespace defaulted, document. Thus, in one respect the two schema approaches are similar, although in another important respect the two schema approaches are very different. Specifically, when all elements are declared globally, it is not possible to take advantage of local names. For example, you can only declare one global element called "title". However, you can locally declare one element called "title" that has a string type, and is a subelement of "book"; And within the same schema (target namespace) you can declare a second element also called "title" that is an enumeration of the values "Mr Mrs Ms".
In Section 2 we explained the basics of XML Schema using a schema that did not declare a target namespace and an instance document that did not declare a namespace. So the question naturally arises: What is the target namespace in these examples and how is it referenced?
In the purchase order schema,
po.xsd, we did not declare a target namespace
for the schema, nor did we declare a prefix (like
po: above) associated with the schema's target
namespace with which we could refer to types and elements
defined and declared within the schema. The consequence of
not declaring a target namespace in a schema is that the
definitions and declarations from that schema, such as
USAddress and purchaseOrder, are
referenced without namespace qualification. In other words
there is no explicit namespace prefix applied to the
references nor is there any implicit namespace applied to
the reference by default. So for example, the
purchaseOrder element is declared using the type
reference PurchaseOrderType. In contrast, all
the XML Schema elements and types used in po.xsd are explicitly qualified with
the prefix xsd: that is associated with the
XML Schema namespace.
Element declarations from a schema with no target
namespace validate unqualified elements in the instance
document. That is, they validate elements for which no
namespace qualification is provided by either an explicit
prefix or by default (xmlns:). So, to validate
a traditional XML 1.0 document which does not use
namespaces at all, you must provide a schema with no target
namespace. Of course, there are many XML 1.0 documents that
do not use namespaces, so there will be many schema
documents written without target namespaces; you must be
sure to give to your processor a schema document that
corresponds to the vocabulary you wish to validate.
The purchase order schema described in Chapter 2 was contained in a single document, and most of the schema constructions-- such as element declarations and type definitions-- were constructed from scratch. In reality, schema authors will want to compose schemas from constructions located in multiple documents, and create new types based on existing types. In this section, we examine mechanisms that enable such compositions and creations.
As schemas become larger, it is often desirable to divide
their content among several schema documents for purposes
such as ease of maintenance, access control, and
readability. For these reasons, we have taken the schema
constructs concerning addresses out of po.xsd, and put them in a new file
called address.xsd.
The modified purchase order schema file is called ipo.xsd:
The file containing the address constructs is:
The various purchase order and
address constructions are now contained in two schema
files, ipo.xsd and
address.xsd. To
include these constructions as part of the international
purchase order schema, in other words to include them in
the international purchase order's namespace, ipo.xsd contains the include element:
<include schemaLocation="http://www.example.com/schemas/address.xsd"/>
The effect of this
include element is to bring in the definitions
and declarations contained in
address.xsd, and make them available as part of
the international purchase order schema target namespace.
The one important caveat to using include is that the target
namespace of the included components must be the same as
the target namespace of the including schema, in this case
http://www.example.com/IPO. Bringing in definitions and
declarations using the
include mechanism effectively adds these components to the
existing target namespace. In Section 4.5, we will describe
a similar mechanism that enables you to modify certain components
when they are brought in.
In our example, we have shown only one including document
and one included document. In practice it is possible to
include more than one document using multiple include
elements, and documents can include documents that
themselves include other documents. However, nesting is
legal only if all the included parts of the schema are
declared with the same target namespace.
Instance documents that conform to schema whose
definitions span multiple schema documents need only
reference the 'topmost' document, and the common namespace,
and it is the responsibility of the processor to gather
together all the definitions specified in the various
included documents. In our example above, the instance
document ipo.xml (see
Section 4.3) references
only the common target namespace,
http://www.example.com/IPO, and the one schema file
http://www.example.com/schemas/ipo.xsd. The
processor is responsible for obtaining the schema file
address.xsd.
In Section 5.4 we describe how schemas can be used to validate content from more than one namespace.
To create our address constructs, we start by creating a
complex type called Address in the usual way
(see address.xsd).
The Address type contains the basic elements
of an address: a name, a street and a city. (Such a
definition will not work for all countries, but it will serve
the purposes of our example.) From this starting point we
derive two new complex types that contain all the elements
of the original type plus additional elements that are
specific to addresses in the US and the UK. The technique
we use here to derive new (complex) address types by
extending an existing type is the same technique we used in
in Section 2.5.1, except
that our base type here is a complex type whereas our base
type in the previous section was a simple type.
We define the two new complex types,
USAddress and UKAddress, using the
complexType
element. In addition, we indicate that the content models of
the new types are complex, i.e. contain elements, by using the
complexContent
element, and we indicate that we are extending the base type
Address by the value of the
base attribute on the
extension element.
When a complex type is derived by extension, its effective
content model is the content model of the base type plus
the content model specified in the type derivation.
Furthermore, the two content models are treated as two
children of a sequential group. In the case of
UKAddress, the content model of
UKAddress is the content model of
Address plus the declarations for a
postcode element and an exportCode
attribute. This is like defining the UKAddress
from scratch as follows:
| Example |
<complexType name="UKAddress">
<sequence>
<!-- content model of Address -->
<element name="name" type="string"/>
<element name="street" type="string"/>
<element name="city" type="string"/>
<!-- appended element declaration -->
<element name="postcode" type="ipo:UKPostcode"/>
</sequence>
<!-- appended attribute declaration -->
<attribute name="exportCode" type="positiveInteger"
use="fixed" value="1"/>
</complexType>
|
In our example scenario, purchase orders are generated in
response to customer orders which may involve shipping and
billing addresses in different countries. The international
purchase order, ipo.xml
below, illustrates one such case where goods are shipped to
the UK and the bill is sent to a US address. Clearly it is
very useful if the schema for international purchase orders
does not have to spell out every possible combination of
international addresses for billing and shipping, and even
more so if we can add new complex types of international
address simply by creating new derivations of
Address.
XML Schema allows us to define the
billTo and shipTo elements as
Address types (see
ipo.xsd) but to use instances of international
addresses in place of instances of Address.
In other words, an instance document whose content conforms
to the UKAddress type will be valid if that
content appears within the document at a location where an
Address is expected (assuming the
UKAddress content itself is valid). To make this
feature of XML Schema work, and to identify exactly which
derived type is intended, the derived type must be
identified in the instance document. The type is identified
using the
xsi:type attribute which is part of the XML
Schema instance namespace. In the example, ipo.xml, use of the
UKAddress and USAddress derived types
is identified through the values assigned to the xsi:type attributes.
In Section 4.8 we'll see how to prevent derived types from being used in this sort of substitution.
In addition to deriving new complex types by extending content models, it is also possible to derive new types by restricting the content models of existing types. Restriction of complex types is conceptually the same as restriction of simple types, except that the restriction of complex types involves a type's declarations rather than the acceptable range of a simple type's values. A complex type derived by restriction is very similar to its base type, except that its declarations are more limited than the corresponding declarations in the base type. In fact, the values represented by the new type are a subset of the values represented by the base type (as is also the case with restriction of simple types). In other words, an application prepared for the values of the base type would not be surprised by the values of the restricted type.
For example, suppose we want to update our definition of
the list of items in an international purchase
order so that it must contain at least one
item on order; The schema shown in ipo.xsd allows an items
element to appear without any child item
elements. To create our new ConfirmedItems
type, we define the new type in the usual way, indicate
that it is derived by restriction from the base type Items,
indicate that we are deriving the new type by restriction,
and provide a new (more restrictive) value for the minimum
number of item element occurrences. Notice that
types derived by restriction must repeat all the components
of the base type definition:
| Deriving ConfirmedItems by Restriction from Items |
<complexType name="ConfirmedItems">
<complexContent>
<restriction base="ipo:Items">
<sequence>
<!-- item element is different than in Items -->
<element name="item" minOccurs="1" maxOccurs="unbounded">
<!-- remainder of definition is same as Items -->
<complexType>
<sequence>
<element name="productName" type="string"/>
<element name="quantity">
<simpleType>
<restriction base="positiveInteger">
<maxExclusive value="100"/>
</restriction>
</simpleType>
</element>
<element name="USPrice" type="decimal"/>
<element ref="ipo:comment" minOccurs="0"/>
<element name="shipDate" type="date" minOccurs="0"/>
</sequence>
<attribute name="partNum" type="ipo:SKU"/>
</complexType>
</element>
</sequence>
</restriction>
</complexContent>
</complexType>
|
This change, requiring at least one child element rather
than allowing zero or more child elements, narrows the
allowable number of child elements from a minimum of 0 to a
minimum of 1. Note that all ConfirmedItems
type elements will also be acceptable as Item
type elements.
To further illustrate restriction, Table 3 shows a number of examples of how element and attribute declarations within type definitions may be restricted (the table shows element syntax although the first three examples are equally valid attribute restrictions).
| Table 3. Restriction Examples | ||
|---|---|---|
| Base | Restriction | Notes |
| default="1" | setting a default value where none was previously given | |
| fixed="100" | setting a fixed value where none was previously given | |
| type="string" | specifying a type where none was previously given | |
| (minOccurs, maxOccurs) | (minOccurs, maxOccurs) | |
| (0, 1) | (0, 0) | deletion of optional component |
| (0, unbounded) | (0, 0) (0, 37) | |
| (1, 9) | (1, 8) (2, 9) (4, 7) (3, 3) | |
| (1, unbounded) | (1, 12) (3, unbounded) (6, 6) | |
| (1, 1) | - | cannot restrict minOccurs or maxOccurs |
In Section 4.1 we described how to
include definitions and declarations obtained from external schema files
having the same target namespace. The include mechanism enables you to use
externally created schema components "as-is", that is, without any
modification. We have just described how to derive new types by extension
and by restriction, and the redefine mechanism we will now describe enables
you to redefine simple and complex types, groups, and attribute groups that
are obtained from external schema files. Like the include mechanism,
redefine requires the external components to be in the same target
namespace as the redefining schema, although external components from
schemas that have no namespace can also be redefined. In the latter cases,
the redefined components become part of the redefining schema's target
namespace.
To illustrate the redefine mechanism, we will use it instead of the
include mechanism in the International Purchase Order schema,
ipo.xsd, and we will use it to modify
the definition of the complex type Address contained in
address.xsd:
| Using redefine in the International Purchase Order |
<schema targetNamespace="http://www.example.com/IPO"
xmlns="http://www.w3.org/2000/08/XMLSchema"
xmlns:ipo="http://www.example.com/IPO>
<!-- bring in address constructs -->
<redefine
schemaLocation="http://www.example.com/schemas/address.xsd">
<!-- redefinition of Address -->
<complexType name="Address">
<complexContent>
<extension base="Address">
<sequence>
<element name="country" type="string"/>
</sequence>
</extension>
</complexContent>
</complexType>
</redefine>
<!-- etc -->
</schema>
|
The redefine element acts very much like the include element, it includes
all the declarations and definitions from the address.xsd
file. The complex type definition of Address uses the familiar extension
syntax to add a country element to the definition of Address. However,
note that the base type is also Address. Outside of the redefine element, any
such attempt to define a complex type with the same name (and in the same
namespace) as the base from which it is being derived would cause an error.
But in this case, there is no error, and the extended definition of Address
becomes the only definition of Address.
Now that Address has been redefined, the extension applies to all schema
components that make use of Address. For example, address.xsd contains
definitions of international address types that are derived from Address.
These derivations will reflect the redefined Address type, as shown in the
following snippet:
| Snippet of ipo.xml using Redefined Address |
.... <shipTo exportCode="1" xsi:type="ipo:UKAddress"> <name>Helen Zoe</name> <street>47 Eden Street</street> <city>Cambridge</city> <!-- country was added to Address which is base type of UKAddress --> <country>United Kingdom</country> <!-- postcode was added as part of UKAddress --> <postcode>CB1 1JR</postcode> </shipTo> .... |
Our example has been carefully constructed so that the redefined Address
type does not conflict in any way with the types that are derived from
the original Address definition. But note that it would be very easy to
create a conflict. For example, if the international address type
derivations had extended Address by adding a country element, then the
redefinition of Address would be adding an element of the same name to
the content model of Address. It is illegal to have two elements of the
same name (and in the same target namespace) but different types in a content model, and so
the attempt to redefine Address would cause an error. In general, redefine
does not protect you from such errors, and it should be used cautiously.
XML Schema provides a mechanism,
called substitution groups, that allows elements to be
substituted for other elements. More specifically, elements
can be assigned to a special class of elements that are
said to be substitutable for a particular named element which
is called the exemplar. Note that the exemplar must be a
global element. For example, we can declare two elements
called customerComment and
shipComment and assign them to a substitution group
whose exemplar is comment, and so
customerComment and shipComment can be
used anyplace that we are able to use
comment. Elements in a substitution group must have
the same type as the examplar, or they can have a type that
has been derived from the exemplar's type. To declare these
two new elements, and to make them substitutable for the
comment element, we use the following syntax:
| Declaring Elements Substitutable for comment |
<element name="shipComment" type="string"
substitutionGroup="ipo:comment" />
<element name="customerComment" type="string"
substitutionGroup="ipo:comment" />
|
When these declarations are added to the international
purchase order schema, comment can be
substituted for in the instance document, for example:
| Snippet of ipo.xml with Substituted Elements |
....
<items>
<item partNum="833-AA">
<productName>Lapis necklace</productName>
<quantity>1</quantity>
<USPrice>99.95</USPrice>
<ipo:shipComment>Use gold wrap if possible</ipo:shipComment>
<ipo:customerComment>
Want this for the holidays!
</ipo:customerComment>
<shipDate>1999-12-05</shipDate>
</item>
</items>
....
|
Note that when an instance document contains element
substitutions whose types are derived from those of their
exemplars, it is not necessary to identify the
derived types using the xsi:type construction that
we described in Section
4.3.
The existence of a substitution group does not require any of the elements in that class to be used, nor does it preclude use of the exemplar. It simply provides a mechanism for allowing elements to be used interchangeably.
XML Schema provides a mechanism to
force substitution for a particular element or type. When
an element or type is declared to be "abstract", it cannot
be used in an instance document. When an element is
declared to be abstract, a member of that element's
substitution group must appear in the instance document.
When an element's corresponding type definition is declared
as abstract, all instances of that element must use
xsi:type to
indicate a derived type that is not abstract.
In the substitution group example we described in Section 4.6, it would be useful to
specifically disallow use of the comment
element so that instances must make use of the
customerComment and shipComment
elements. To declare the comment element
abstract, we modify its original declaration in the
international purchase order schema, ipo.xsd, as follows:
<element name="comment" type="string" abstract="true"/>
With comment declared as abstract, instances
of international purchase orders are now only valid if they
contain customerComment and
shipComment elements.
Declaring an element as abstract requires the use of a
substitution group. Declaring a type as abstract simply
requires the use of a type derived from it (and identified
by the
xsi:type attribute) in the instance document.
Consider the following schema definition:
| Schema for Vehicles |
<schema xmlns="http://www.w3.org/2000/08/XMLSchema" targetNamespace="http://cars.example.com/schema" xmlns:target="http://cars.example.com/schema"> <complexType name="Vehicle" abstract="true"/> <complexType name="Car"> <complexContent> <extension base="target:Vehicle"/> </complexContent> </complexType> <complexType name="Plane"> <complexContent> <extension base="target:Vehicle"/> </complexContent> </complexType> <element name="transport" type="target:Vehicle"/> </schema> |
The transport element is not abstract,
therefore it can appear in instance documents. However,
because its type definition is abstract, it may never
appear in an instance document without an xsi:type attribute that
refers to a derived type. That means the following is not
schema-valid:
<transport xmlns="http://cars.example.com/schema" />
because the transport element's type is
abstract. However, the following is schema-valid:
<transport xmlns="http://cars.example.com/schema"
xmlns:xsi="http://www.w3.org/1999/XMLSchema-instance"
xsi:type="Car"/>
|
because it uses a non-abstract type that is substitutable
for Vehicle.
So far, we have been able to derive new types and use them in instance documents without any restraints. In reality, schema authors will sometimes want to control derivations of particular types, and the use of derived types in instances.
XML Schema provides a couple of mechanisms that control
the derivation of types. One of these mechanisms allows the schema author
to specify that for a particular (simple or
complex) type, new types may not be derived from it, either
(a) by restriction, (b) by extension, or (c) at all. To
illustrate, suppose we want to prevent any derivation of
the Address type by restriction because we
intend for it only to be used as the base for extended
types such as USAddress and
UKAddress. To prevent any such derivations, we
slightly modify the original definition of
Address as follows:
| Preventing Derivations by Restriction of Address |
<complexType name="Address" final="restriction"> <sequence> <element name="name" type="string"/> <element name="street" type="string"/> <element name="city" type="string"/> </sequence> </complexType> |
The restriction value of
the final
attribute prevents derivations by restriction. Preventing
derivations at all, or by extension, are indicated by the
values #all and extension
respectively. There exists also an optional finalDefault attribute
on the schema
element whose value can be one of the values allowed for
the final
attribute. The effect of specifying the finalDefault attribute
is equivalent to specifying a final attribute on every type
definition and element declaration in the schema.
Another type-derivation mechanism controls which facets
can be applied in the derivation of a new simple type. When a simple type
is defined, the fixed
attribute may be applied to any of its facets to prevent a derivation
of that type from modifying the value of the fixed facets. For example,
we can define a Postcode simple type as:
| Preventing Changes to Simple Type Facets |
<simpleType name="Postcode">
<restriction base="string">
<length value="7" fixed="true"/>
</restriction>
</simpleType>
|
Once this simple type has been defined, we can derive a new postal code type in which we apply a facet not fixed in the base definition, for example:
| Legal Derivation from Postcode |
<simpleType name="UKPostcode">
<restriction base="ipo:Postcode">
<pattern value="[A-Z]{2}\d\s\d[A-Z]{2}"/>
</restriction>
</simpleType>
|
However, we cannot derive a new postal code in which we re-apply any facet that was fixed in the base definition:
| Illegal Derivation from Postcode |
<simpleType name="UKPostcode">
<restriction base="ipo:Postcode">
<pattern value="[A-Z]{2}\d\d[A-Z]{2}"/>
<!-- illegal attempt to modify facet fixed in base type -->
<length value="6" fixed="true"/>
</restriction>
</simpleType>
|
In addition to the mechanisms that control type
derivations, XML Schema provides a mechanism that controls which
derivations and substitution groups may
be used in instance documents. In Section 4.3, we described how the
derived types, USAddress and
UKAddress, could be used by the shipTo
and billTo elements in instance documents.
These derived types can replace the content model provided
by the Address type because they are derived
from the Address type. However, replacement by
derived types can be controlled using the block attribute in a type
definition. For example, if we want to block any
derivation-by-restriction from being used in place of
Address (perhaps for the same reason we
defined Address with final="restriction"), we can
modify the original definition of Address as
follows:
| Preventing Derivations by Restriction of Address in the Instance |
<complexType name="Address" block="restriction"> <sequence> <element name="name" type="string"/> <element name="street" type="string"/> <element name="city" type="string"/> </sequence> </complexType> |
The restriction value on
the block
attribute prevents derivations-by-restriction from
replacing Address in an instance. However, it
would not prevent UKAddress and
USAddress from replacing Address
because they were derived by extension. Preventing
replacement by derivations at all, or by
derivations-by-extension, are indicated by the values
#all and extension respectively.
As with final,
there exists also an optional blockDefault attribute
on the schema
element whose value can be one of the values allowed for
the block
attribute. The effect of specifying the blockDefault attribute
is equivalent to specifying a block attribute on every type
definition and element declaration in the schema.
The home-products ordering and billing application can
generate ad-hoc reports that summarise how many of which
types of products have been billed on a per region basis.
An example of such a report, one that covers the fourth
quarter of 1999, is shown in
4Q99.xml.
Notice that in this section we use qualified elements in the schema, and default namespaces where possible in the instances.
The report lists, by number and quantity, the parts billed to various zip codes, and it provides a description of each part mentioned. In summarising the billing data, the intention of the report is clear and the data is unambiguous because a number of constraints are in effect. For example, each zip code appears only once (uniqueness constraint). Similarly, the description of every billed part appears only once although parts may be billed to several zip codes (referential constraint), see for example part number 455-BX. In the following sections, we'll see how to specify these constraints using XML Schema.
XML Schema enables us to indicate
that any attribute or element value must be unique within a
certain scope. To indicate that one particular attribute or
element value is unique, we use the unique element first to
"select" a set of elements, and then to identify the
attribute or element "field" relative to each selected
element that has to be unique within the scope of the set
of selected elements. In the case of our report schema,
report.xsd, the
selector
element contains an XPath expression (see XML Path Language 1.0),
regions/zip, that selects a list of all the
zip elements in a report instance, and the
field element
contains a second XPath expression, @code,
that specifies that the code attribute values
of those elements must be unique. Note that the XPath
expressions limit the scope of what must be unique. The
report might contain another code attribute,
but its value does not have to be unique because it lies
outside the scope defined by the XPath expressions.
Moreover, we can also indicate combinations of fields that
must be unique. To illustrate, suppose we can relax the
constraint that zip codes may only be listed once, although
we still want to enforce the constraint that any product is
listed only once within a given zip code. We could achieve
such a constraint by specifying that the combination of zip
code and product number must be unique. From the report
document, 4Q99.xml,
the combined values of zip code and
number would be: {95819 872-AA}, {95819 926-AA},
{95819 833-AA}, {95819 455-BX}, and {63143 455-BX}.
Clearly, these combinations do not distinguish between zip
code and number combinations
derived from single or multiple listings of any particular
zip, but the combinations would unambiguously represent a
product listed more than once within a single zip. In other
words, a schema processor could detect violations of the
uniqueness constraint.
To define combinations of values, we simply add field elements to identify
all the values involved. So, to add the part number value
to our existing definition, we add a new field element whose XPath
expression, part/@number, identifies the
number attribute of part elements
that are children of the zip elements
identified by regions/zip:
| A Unique Composed Value |
<unique name="dummy1"> <selector>regions/zip</selector> <field>@code</field> <field>part/@number</field> </unique> |
In the 1999 quarterly report, the
description of every billed part appears only once. We
could enforce this constraint using unique, however, we also want
to ensure that every part-quantity element listed under a
zipcode has a corresponding part description. We enforce
the constraint using the
key and
keyRef elements. the report schema, report.xsd, shows that the
key and keyRef constructions are
applied using almost the same syntax as unique. The key element
applies to the number attribute value of
part elements that are children of the
parts element. This declaration of
number as a key means that its value must be unique
and not nullable, and the name that is associated with the
key, pNumKey, makes the key referenceable from
elsewhere.
To ensure that the part-quantity elements have
corresponding part descriptions, we say that the
number attribute (
<field>@number</field>) of those
elements
(<selector>regions/zip/part</selector>)
must reference the pNumKey key. This
declaration of number as a keyRef does not
mean that its value must be unique, but it does mean there
must exist a pNumKey with the same value.
As you may have figured out by analogy with unique, it is possible to
define combinations of
key and
keyRef values. Using this mechanism, we could go
beyond simply requiring the product numbers to be equal,
and define a combination of values that must be equal. Such
values may involve combinations of multiple value types
(string, integer,
date, etc), provided that the order and type of
the field element
references is the same in both the key and keyRef definitions.
XML 1.0 provides a mechanism for ensuring uniqueness using
the ID attribute and its associated attributes IDREF and
IDREFS. This mechanism is also provided in XML Schema
through the ID, IDREF, and IDREFS simple types which can be used
for declaring XML 1.0-style attributes. XML Schema also
introduces new mechanisms that are more flexible and
powerful. For example, XML Schema's mechanisms can be
applied to any element and attribute content, regardless of
its type. In contrast, ID is a type of attribute
and so it cannot be applied to attributes, elements or
their content. Furthermore, Schema enables you to specify
the scope within which uniqueness applies whereas the scope
of an ID is fixed to be the whole document. Finally, Schema
enables you to create
keys or a
keyRef from combinations of element and
attribute content whereas ID has no such facility.
The report schema, report.xsd, makes use of the
simple type xipo:SKU that is defined in
another schema, and more specifically, in another target
namespace. Recall that we used include so that the schema in
ipo.xsd could make use
of definitions and declarations from address.xsd. We cannot use
include here because
it can only pull in definitions and declarations from a
schema whose target namespace is the same as the including
schema's target namespace. Hence, the include element does not
identify a namespace (although it does require a
schemaLocation). The import mechanism that we
describe in this section is an important mechanism that
enables schema components from different target namespaces
to be used together, and hence enables the schema
validation of instance content defined across multiple
namespaces.
To import the type SKU
and use it in the report schema, we identify the namespace
in which SKU is defined, and associate that
namespace with a prefix for use in the report schema.
Concretely, we use the
import element to identify SKU's
target namespace (http://www.example.com/IPO),
and we associate the namespace with the prefix
xipo using a standard namespace declaration. The
simple type SKU, defined in the namespace
http://www.example.com/IPO, may then be
referenced as xipo:SKU in any of the report
schema's definitions and declarations.
In our example, we imported one simple type from one
external namespace, and used it for declaring attributes.
XML Schema in fact permits multiple schema components to be
imported, from multiple namespaces, and they can be
referred to in both definitions and declarations. For
example in
report.xsd we could additionally reuse the
comment element declared in ipo.xsd by referencing that element
in a declaration:
<element ref="xipo:comment"/>
Note however, that we cannot reuse the shipTo
element from po.xsd, and
the following is not legal because only global schema
components can be imported:
<element ref="xipo:shipTo"/>
In ipo.xsd,
comment is declared as a global element, in other
words it is declared as an element of the schema. In contrast,
shipTo is declared locally, in other words it is an
element declared inside a complex type definition,
specifically the PurchaseOrderType type.
Complex types can also be imported, and they can be used
as the base types for deriving new types. Only named
complex types can be imported; Local, anonymously defined
types cannot. Suppose we want to include in our reports the
name of an analyst, along with contact information. We can
reuse the (globally defined) complex type
USAddress from
address.xsd, and extend it to define a new type
called Analyst by adding the new elements
phone and email:
| Defining Analyst by Extending USAddress |
<complexType name="Analyst">
<complexContent>
<extension base="xipo:USAddress">
<sequence>
<element name="phone" type="string"/>
<element name="email" type="string"/>
</sequence>
</extension>
</complexContent>
</complexType>
|
Using this new type we declare an element called
analyst as part of the purchaseReport
element declaration (declarations not shown) in the report
schema. Then, the following instance document would conform
to the modified report schema:
| Instance Document Conforming to Report Schema with Analyst Type |
<purchaseReport
xmlns="http://www.example.com/Report"
period="P3M" periodEnding="1999-12-31">
<!-- regions and parts elements omitted -->
<analyst>
<name>Wendy Uhro</name>
<street>10 Corporate Towers</street>
<city>San Jose</city>
<state>CA</state>
<zip>95113</zip>
<phone>408-271-3366</phone>
<email>uhro@example.com</email>
</analyst>
</purchaseReport>
|
When schema components are imported from multiple
namespaces, each namespace must be identified with a
separate import
element. The
import elements themselves must appear as the
first children of the
schema element. Furthermore, each namespace must
be associated with a prefix, using a standard namespace
declaration, and that prefix used to qualify references to
any schema components belonging to that namespace. Finally,
import elements
optionally contain a schemaLocation
attribute to help locate resources associated with the
namespaces. We discuss the schemaLocation
attribute in more detail in a later section.
As XML schemas become more widespread, schema authors will want to create simple and complex types that can be shared and used as the basic building blocks for building new schemas. XML Schemas already provides types that play this role: the simple types summarised in Appendix B. However, schema authors will want to go beyond this basic collection of simple data types, and create libraries of types to represent currency, units of measurement, business addresses, and so on. Each library might consist of a schema containing one or more definitions, for example, a schema containing a currency type:
| Example Currency Type in Type Library |
<schema targetNamespace="http://www.example.com/Currency"
xmlns:c="http://www.example.com/Currency"
xmlns="http://www.w3.org/2000/08/XMLSchema">
<annotation>
<documentation>
Definition of Currency type based on ISO 4217
</documentation>
</annotation>
<complexType name="Currency">
<simpleContent>
<extension base="decimal">
<attribute name="name">
<simpleType>
<restriction base="string">
<enumeration value="AED">
<annotation>
<documentation>
United Arab Emirates: Dirham (1 Dirham = 100 Fils)
</documentation>
</annotation>
</enumeration>
<enumeration value="AFA">
<annotation>
<documentation>
Afghanistan: Afghani (1 Afghani = 100 Puls)
</documentation>
</annotation>
</enumeration>
<enumeration value="ALL">
<annotation>
<documentation>
Albania, Lek (1 Lek = 100 Qindarka)
</documentation>
</annotation>
</enumeration>
<!-- and other currencies -->
</restriction>
</simpleType>
</attribute>
</extension>
</simpleContent>
</complexType>
</schema>
|
An example of an element appearing in an instance and having this type:
<convertFrom name="AFA"/>199.37</convertFrom>
Once we have defined the currency type, we can make it available for re-use
in other schemas through the import
mechanism just described.
In previous sections we have seen several mechanisms for extending the content models of complex types. For example, a mixed content model can contain arbitrary character data in addition to elements, and for example, a content model can contain particular elements whose types are imported from external namespaces. However, these mechanisms provide very broad and very narrow controls respectively. The purpose of this section is to describe a flexible mechanism that enables content models to be extended by any elements and attributes belonging to specified namespaces.
To illustrate, consider a version of the quarterly report,
4Q99html.xml, in
which we have embedded an HTML representation of the XML
parts data. The HTML content appears as the content of the
element htmlExample, and the default
namespace is changed on the outermost HTML element
(table) so that all the HTML elements belong
to the HTML namespace,
http://www.w3.org/1999/xhtml:
To permit the appearance of HTML in
the instance document we modify the report schema by
declaring a new element htmlExample whose
content is defined by the
any element. In general, an any element specifies that any
well-formed XML is permissible in a type's content model.
In the example, we require the XML to belong to the
namespace http://www.w3.org/1999/xhtml, in
other words, it should be HTML. The example also requires
there to be at least one element present from this
namespace, as indicated by the values of minOccurs and maxOccurs:
| Modification to purchaseReport Declaration to Allow HTML in Instance |
<element name="purchaseReport">
<complexType>
<sequence>
<element name="regions" type="r:RegionsType"/>
<element name="parts" type="r:PartsType"/>
<element name="htmlExample">
<complexType>
<sequence>
<any namespace="http://www.w3.org/1999/xhtml"
minOccurs="1" maxOccurs="unbounded"
processContents="skip"/>
</sequence>
</complexType>
</element>
</sequence>
<attribute name="period" type="timeDuration"/>
<attribute name="periodEnding" type="date"/>
</complexType>
</element>
|
The modification permits some
well-formed XML belonging to the namespace
http://www.w3.org/1999/xhtml to appear inside the
htmlExample element. Therefore 4Q99html.xml is permissible
because there is one element which (with its children) is
well formed, the element appears inside the appropriate
element (htmlExample), and the instance
document asserts that the element and its content belongs
to the required namespace. However, the HTML may not
actually be valid because nothing in 4Q99html.xml by itself can
provide that guarantee. If such a guarantee is required,
the value of the processContents
attribute should be set to strict (which is in
fact the default value). In this case, an XML processor is
obliged to obtain the schema associated with the required
namespace, and validate the HTML appearing within the
htmlExample element. Alternatively, the value
of the
processContents attribute can be set to
lax, in which case the processor will validate the
HTML on a can-do basis: It will validate elements and
attributes for which it can obtain schema information, but
it will not signal errors for those it cannot obtain schema
information.
Namespaces may be used to permit and
forbid element content in various ways depending upon the
value of the
nameSpace attribute, as shown in Table 4:
In addition to the any element which enables element
content according to namespaces, there is a corresponding
anyAttribute element which enables attributes to
appear in elements. For example, we can permit any HTML
attribute to appear as part of the htmlExample
element by adding
anyAttribute to its declaration:
| Modification to htmlExample Declaration to Allow HTML Attributes |
<element name="htmlExample">
<complexType>
<sequence>
<any namespace="http://www.w3.org/1999/xhtml"
minOccurs="1" maxOccurs="unbounded"
processContents="skip"/>
</sequence>
<anyAttribute namespace="http://www.w3.org/1999/xhtml"/>
</complexType>
</element>
|
This declaration permits an HTML attribute, say
href, to appear in the htmlExample
element. For example:
| An HTML attribute in the htmlExample Element |
....
<htmlExample xmlns:h="http://www.w3.org/1999/xhtml"
h:href="http://www.example.com/reports/4Q99.html">
<!-- HTML markup here -->
</htmlExample>
....
|
The
nameSpace attribute in an anyAttribute element can
be set to any of the values listed in
Table 4 for the
any element, and anyAttribute can be
specified with a processContents
attribute. In contrast to an
any element, anyAttribute cannot
constrain the number of attributes that may appear in an
element.
XML Schema uses the schemaLocation and
xsi:schemaLocation attributes in three
circumstances.
1. In an instance document, the
attribute
xsi:schemaLocation provides hints from the
author to a processor regarding the location of schema
documents. The author warrants that these schema documents
are relevant to checking the validity of the document
content, on a namespace by namespace basis. For example, we
can indicate the location of the Report schema to a
processor of the Quarterly Report:
| Using schemaLocation in the Quarterly Report, 4Q99html.xml |
<purchaseReport xmlns="http://www.example.com/Report" xmlns:xsi="http://www.w3.org/1999/XMLSchema-instance" xsi:schemaLocation="http://www.example.com/Report http://www.example.com/Report.xsd" period="P3M" periodEnding="1999-12-31"> <!-- etc --> </purchaseReport> |
The
schemaLocation attribute contains pairs of
values: The first member of each pair is the namespace for
which the second member is the hint describing where to
find to an appropriate schema document. The presence of
these hints does not require the processor to obtain or use
the cited schema documents, and the processor is free to
use other schemas obtained by any suitable means, or to use
no schema at all.
A schema is not required to have a namespace (see Section 3.4) and so there is a
noNamespaceSchemaLocation attribute which is
used to provide hints for the locations of schema documents
that do not have target namespaces.
2. In a schema, the
include element has a required schemaLocation
attribute, and it contains a URI reference which must
identify a schema document. The effect is to compose a
final effective schema by merging the declarations and
definitions of the including and the included schemas. For
example, in Section 4, the type
definitions of Address,
USAddress, UKAddress,
USState (along with their attribute and local
element declarations) from
address.xsd were added to the element
declarations of purchaseOrder and
comment, and the type definitions of
PurchaseOrderType, Items and
SKU (along with their attribute and local element
declarations) from
ipo.xsd to create a single schema.
3. Also in a schema, the
import element has optional nameSpace and schemaLocation
attributes. If present, the schemaLocation
attribute is understood in a way which parallels the
interpretation of
xsi:schemaLocation in (1). Specifically, it
provides a hint from the author to a processor regarding
the location of a schema document that the author warrants
supplies the required components for the namespace
identified by the
nameSpace attribute. To import components that
are not in any target namespace, the import element is used without
a nameSpace
attribute (and with or without a schemaLocation
attribute). References to components imported in this
manner are unqualified.
Note that the
schemaLocation is only a hint and some
processors and applications will have reasons to not use
it; For example, an HTML editor may have a built-in HTML
schema.
An instance document may be processed against a schema to verify whether the rules specified in the schema are honored in the instance. Typically, such processing actually does two things, (1) it checks for conformance to the rules, a process called schema validation, and (2) it adds supplementary information that is not immediately present in the instance, such as types and default values, called infoset contributions.
The author of an instance document, such as a particular
purchase order, may claim, in the instance itself, that it
conforms to the rules in a particular schema. The author
does this using the schemaLocation
attribute discussed above. But regardless of whether a
schemaLocation attribute is present, an
application is free to process the document against any
schema. For example, a purchasing application may have the
policy of always using a certain purchase order schema,
regardless of any schemaLocation
values.
Conformance checking can be thought of as proceeding in steps, first checking that the root element of the document instance has the right contents, then checking that each subelement conforms to its description in a schema, and so on until the entire document is verified. Processors are required to report what checking has been carried out.
To check an element for conformance, the processor first
locates the declaration for the element in a schema, and
then checks that the targetNamespace
attribute in the schema matches the actual namespace URI of
the element (or, alternatively, that the schema does not
have a
targetNamespace attribute and the instance
element is not namespace-qualified).
Supposing the namespaces match, the processor then
examines the type of the element, either as given by the
declaration in the schema, or by an xsi:type attribute in the
instance. If the latter, the instance type must be an
allowed substitution for the type given in the schema; what
is allowed is controlled by the block attribute in the
element declaration. At this same time, default values and
other infoset contributions are applied.
Next the processor checks the immediate attributes and
contents of the element, comparing these against the
attributes and contents permitted by the element's type.
For example, considering a shipTo element such
as the one in Section 2.1, the
processor checks what is permitted for an
Address, because that is the shipTo
element's type.
If the element has a simple type, the processor verifies that the element has no attributes or contained elements, and that its character content matches the rules for the simple type. This sometimes involves checking the character sequence against regular expressions or enumerations, and sometimes it involves checking that the character sequence represents a value in a permitted range.
If the element has a complex type, then the processor checks that any required attributes are present and that their values conform to the requirements of their simple types. It also checks that all required subelements are present, and that the sequence of subelements (and any mixed text) matches the content model declared for the complex type. Regarding subelements, schemas can either require exact name matching, permit substitution by an equivalent element or permit substitution by any element allowed by an 'any' particle.
Unless a schema indicates otherwise (as it can for 'any' particles) conformance checking then proceeds one level more deeply by looking at each subelement in turn, repeating the process described above.
Many people have contributed ideas, material and feedback that has improved this document. In particular, the editor would like to acknowledge contributions from David Beech, Paul Biron, Don Box, Allen Brown, David Cleary, Dan Connolly, Roger Costello, Martin Gudgin, Dave Hollander, Joe Kesselman, John McCarthy, Andrew Layman, Eve Maler, Ashok Malhotra, Noah Mendelsohn, Henry Thompson, and Priscilla Walmsley for validating the examples.
The legal values for each simple type can be constrained through the application of one or more facets. Tables B1.a, B1.b and B1.c list all of XML Schemas built-in simple types and the facets applicable to each type. The names of the simple types and the facets are linked from the tables to the corresponding descriptions in XML Schema Part 2: Datatypes
| Table B1.a. Simple Types & Applicable Facets | |||||
|---|---|---|---|---|---|
| Simple Types | Facets | ||||
| length | minLength | maxLength | pattern | enumeration | |
| string | y | y | y | y | y |
| byte | y | y | |||
| unsignedByte | y | y | |||
| binary | y | y | y | y | |
| integer | y | y | |||
| positiveInteger | y | y | |||
| negativeInteger | y | y | |||
| nonNegativeInteger | y | y | |||
| nonPositiveInteger | y | y | |||
| int | y | y | |||
| unsignedInt | y | y | |||
| long | y | y | |||
| unsignedLong | y | y | |||
| short | y | y | |||
| unsignedShort | y | y | |||
| decimal | y | y | |||
| float | y | y | |||
| double | y | y | |||
| boolean | y | ||||
| time | y | y | |||
| timeInstant | y | y | |||
| timePeriod | y | y | |||
| timeDuration | y | y | |||
| date | y | y | |||
| month | y | y | |||
| year | y | y | |||
| century | y | y | |||
| recurringDay | y | y | |||
| recurringDate | y | y | |||
| recurringDuration | y | y | |||
| Name | y | y | y | y | y |
| QName | y | y | y | y | y |
| NCName | y | y | y | y | y |
| uriReference | y | y | y | y | y |
| language | y | y | y | y | y |
| ID | y | y | y | y | y |
| IDREF | y | y | y | y | y |
| IDREFS | y | y | y | y | |
| ENTITY | y | y | y | y | y |
| ENTITIES | y | y | y | y | |
| NOTATION | y | y | y | y | y |
| NMTOKEN | y | y | y | y | y |
| NMTOKENS | y | y | y | y | |
The facets listed in Table B1.b apply only to simple types which are ordered. Not all simple types are ordered and so B1.b does not list all of the simple types.
| Table B1.b. Simple Types & Applicable Facets | ||||||||
|---|---|---|---|---|---|---|---|---|
| Simple Types | Facets | |||||||
|
max Inclusive |
max Exclusive |
min Inclusive |
min Exclusive |
precision | scale | encoding | ||
| byte | y | y | y | y | y | y | ||
| unsignedByte | y | y | y | y | y | y | ||
| binary | y | |||||||
| integer | y | y | y | y | y | y | ||
| positiveInteger | y | y | y | y | y | y | ||
| negativeInteger | y | y | y | y | y | y | ||
| nonNegativeInteger | y | y | y | y | y | y | ||
| nonPositiveInteger | y | y | y | y | y | y | ||
| int | y | y | y | y | y | y | ||
| unsignedInt | y | y | y | y | y | y | ||
| long | y | y | y | y | y | y | ||
| unsignedLong | y | y | y | y | y | y | ||
| short | y | y | y | y | y | y | ||
| unsignedShort | y | y | y | y | y | y | ||
| decimal | y | y | y | y | y | y | ||
| float | y | y | y | y | ||||
| double | y | y | y | y | ||||
| time | y | y | y | y | ||||
| timeInstant | y | y | y | y | ||||
| timePeriod | y | y | y | y | ||||
| timeDuration | y | y | y | y | ||||
| date | y | y | y | y | ||||
| month | y | y | y | y | ||||
| year | y | y | y | y | ||||
| century | y | y | y | y | ||||
| recurringDay | y | y | y | y | ||||
| recurringDate | y | y | y | y | ||||
| recurringDuration | y | y | y | y | ||||
As shown in Table B1.c, the period and duration facets apply only to temporal simple types.
| Table B1.c. Simple Types & Applicable Facets | ||||||||
|---|---|---|---|---|---|---|---|---|
| Simple Types | Facets | |||||||
| period | duration | |||||||
| time | y | y | ||||||
| timeInstant | y | y | ||||||
| timePeriod | y | y | ||||||
| timeDuration | ||||||||
| date | y | y | ||||||
| month | y | y | ||||||
| year | y | y | ||||||
| century | y | y | ||||||
| recurringDay | y | y | ||||||
| recurringDate | y | y | ||||||
| recurringDuration | y | y | ||||||
XML Schema's
pattern facet uses a regular expression language
that supports
Unicode. The language is similar to the regular
expression language used in the Perl
Programming language, although expressions are matched
against entire lexical representations rather than
user-scoped lexical representions such as line and
paragraph. For this reason, the expression language does
not contain the metacharacters ^ and $, although ^ is used
to express exception, e.g. [^0-9]x.
| Table C1. Examples of Regular Expressions | |
|---|---|
| Expression | Match(s) |
| Chapter \d | Chapter 0, Chapter 1, Chapter 2 .... |
| Chapter\s\d | Chapter followed by a single whitespace character (space, tab, newline, etc), followed by a single digit |
| Chapter\s\w | Chapter followed by a single whitespace character (space, tab, newline, etc), followed by a word character (XML 1.0 Letter or Digit) |
| Española | Española |
| \p{Lu} | any uppercase character, the value of \p{} (e.g. "Lu") is defined by Unicode |
| \p{IsGreek} | any Greek character, the 'Is' construction may be applied to any block name (e.g. "Greek") as defined by Unicode |
| \P{IsGreek} | any non-Greek character, the 'Is' construction may be applied to any block name (e.g. "Greek") as defined by Unicode |
| a*x | x, ax, aax, aaax .... |
| a?x | ax, x |
| a+x | ax, aax, aaax .... |
| (a|b)+x | ax, bx, aax, abx, bax, bbx, aaax, aabx, abax, abbx, baax, babx, bbax, bbbx, aaaax .... |
| [abcde]x | ax, bx, cx, dx, ex |
| [a-e]x | ax, bx, cx, dx, ex |
| [-ae]x | -x, ax, ex |
| [ae-]x | ax, ex, -x |
| [a-e-[bd]]x | ax, cx, ex |
| [^0-9]x | any non-digit character followed by the character x |
| \Dx | any non-digit character followed by the character x |
| .x | any character followed by the character x |
| .*abc.* | 1x2abc, abc1x2, z3456abchooray .... |
| ab{2}x | abbx |
| ab{2,4}x | abbx, abbbx, abbbbx |
| ab{2,}x | abbx, abbbx, abbbbx .... |
| (ab){2}x | ababx |
XML Schema Elements. Each element name is linked to a formal XML description in either the Structures or Datatypes parts of the XML Schema specification. Element names are followed by one or more links to examples (identified by section number) in the Primer.
all: 2.7
annotation: 2.6
any: 5.5
anyAttribute: 5.5
appInfo: 2.6
attribute: 2.2
attributeGroup: 2.8
choice: 2.7
complexContent: 2.5.3
complexType: 2.2
documentation: 2.6
element: 2.2
enumeration: 2.3
extension:
field: 5.1
group: 2.7
import: 5.4
include: 4.1
key: 5.2
keyRef: 5.2
length: 2.3.1
list: 2.3.1
maxInclusive: 2.3
maxLength: 2.3.1
minInclusive: 2.3
minLength: 2.3.1
pattern: 2.3
redefine: 4.5
restriction: 2.3,
4.4
schema: 2.1
selector: 5.1
sequence: 2.7
simpleContent: 2.5.1
simpleType: 2.3
union: 2.3.2
unique: 5.1
XML Schema Attributes. Each attribute name is followed by one or more pairs of references. Each pair of references consists of a link to an example in the Primer, plus a link to a formal XML description in either the Structures or Datatypes parts of the XML Schema specification.
abstract:
element declaration [Structures],
complex type definition [Structures]
attributeFormDefault:
schema element [Structures]
base: simple type definition [Datatypes],
complex type definition [Structures]
block: complex type definition [Structures],
blockDefault: schema
element [Structures],
elementFormDefault:
schema element [Structures]
final: complex type definition [Structures]
finalDefault: schema
element [Structures]
fixed: simple type definition [Datatypes]
form: element declaration [Structures],
attribute declaration [Structures]
itemType: list type definition [Datatypes]
memberTypes: union type definition [Datatypes]
maxOccurs: element declaration
[Structures]
minOccurs: element declaration
[Structures]
mixed: complex type definition
[Structures]
name: element declaration [Structures],
attribute declaration [Structures],
complex type definition [Structures],
simple type definition [Datatypes]
nameSpace: any
element [Structures],
include element [Structures]
noNamespaceSchemaLocation:
instance element [Structures]
xsi:null:
instance element [Structures]
nullable:
element declaration [Structures]
processContents: any
element [Structures],
anyAttribute element
[Structures]
ref: element declaration [Structures]
schemaLocation: include
specification [Structures],
redefine
specification [Structures],
import specification [Structures]
xsi:schemaLocation: instance
attribute [Structures]
substitutionGroup: element
declaration [Structures]
targetNamespace:
schema element [Structures]
type: element declaration [Structures],
attribute declaration [Structures]
xsi:type:
instance element [Structures]
use: attribute declaration [Structures]
value: attribute declaration [Structures],
facet specification
XML Schema's simple types are described in Table 2.